The use of oxygen in respiratory therapy is fundamental and requires oxygen delivery at the location where the therapy is to be administered. Although oxygen is readily available at most institutions and hospitals in substantial quantity, for home care patients, oxygen is normally delivered in compressed oxygen cylinders on an "as needed" basis.
When compressed oxygen is used, it is withdrawn from a cylinder or other source via a regulator valve assembly at a prescribed and and definite flow rate. However, the user does not breathe all of the oxygen delivered; instead, less than half of the oxygen is actually inspired during the breathing cycle. As a result, a substantial amount of the oxygen is wasted and simply vented to atmosphere, obviously at a significant economic disadvantage.
To eliminate or reduce such a loss, a number of systems have been proposed including those described in U.S. Pat. No. 4,054,133 and referred to in "Chest", Vol. 74, July 1978, and U.S. Pat. Nos. 4,120,300, 4,278,110, and 4,381,002. The earliest patent refers to a pneumatic system incorporating a plurality of conduits, control chambers, and diaphragms for sensing pressure differences from a cannula during a user's respiratory cycle. The latter patents incorporate fluidic elements in combination with valves for controlling oxygen flow from a source.